Propeller blade



Jul 29, 1924. J. W. SMITH 'PRO PELLER BLADE FiledMarch 16. 1918 2 sheets shet 1 Patented 3y as, are.

rA-ENr OFFICE.

JOHN W. SMITH, E -I1Ii #ELPHIA, PENNSYLVANIA.

rzaorannna BE.

Application filed march 16-, 1918. Serial No. 222,932.

To all whom it may concern: Be it known that I, JOHN W. Smm, a

citizen of the United States, residing in,

of form are important, and where'rotation at a very high speed is contemplated.

One of the ob ecth of the invention is to permit the use of relatively thin sheet metal as the material for the blade, and, since the blade is hollow, tb dispose the sheet metal in such structural form as will 'mosteffectively resist distortion under :the strains .characteristic of use. Furthermore, the invention has the two-fold aspect that whilst my improved blade itself possesses the desired qualities, its manufacture is very economicaI, with the requisite accuracy of external contour and proper structural disposition of the metal to ensure such qualities.

In the accompanying drawings, Fig. I, represents, in longitudinal section, a preferred form of tubular metallic blank which I have found advantageous for the process. I Figs. II, III, and V, are diagrammatic views of a typical blade, embodying my invention; Fig. II, being a side elevation in isometric erspective, and Fig. III, an'edge elevation n isometric perspective, the spiral contour, or twist, of the blade being ignored in both instances, in order to avoid oomph-- a0 cation. Fig. IV, represents a series of transverse sections, of the blade upon planes corresponding with prolon ations of the successive transverse dot and ash lines in Figs. II, and IILthe twist of the blade being also ignored in these sectional figures.

the art that these three figures are intended only to exemplify certain particular structural features, and that the blade itself will embody further details of external and internal contour, such as those due to the twist of the blade, which are entirely consistent with thoseshown, but which are not repre It will be, understood'by those skilled in- Figs. V, and VI, are partial sectional views on an enlarged scale, representing a preferred mounting for the blades in a. practical embodiment for use in an air-craft; Fig. V, being asection on the line of the axis of the driving shaft, and Fig. VI, a transverse section and partial side view on the lines VI, VI, of Fig. V.

' Fig. VII, represents a transverse section,

on an enlarged scale, through a propeller blade embodying my invention, illustrating certain adjunctive devices which I prefer to employ for ensuring permanence of form.

- The preferred initial blank for my novel propeller blade is a steel tube 1, which may be originally formed with a circumferential flange 2, at one extremity, dr said flange may be formed duringthe process of treatment.

The wall of the tube is uniformly tapered, as

indicated, the taper being preferably upon the outside. The blank may either be initially formed as an integral tube, or may be formed by welding, butshould be substantially seamless.

In order to convert this tube into a blade of the desired character, I employ dies which may either shape the tube by a series of operations, or by one. direct operation, the internal cavity of said dies having a contour which corresponds to the ultimate desired form. ,The dies will therefore be of such character as to impart to the blank not only the particular structural features indicated in Figs. II, III, and IV, but such further details as may be contemplated, as, for instance, a spiral contour, or axial twist of the blade.

In order to accomplish the necessary modification of the tube 1, without wrinkles or-distortion, I fill it before the conclusion of the die-pressing operation with a core of wax, or similar lastic material, of sufficient hardness to ow onlyv under heavy pres sure ata temperature characteristic 0t the operation. The blank may be charged with the wax, etc. in any convenient manner before the pressing is begun, or the shaping of the blank may be partiallyaccomplished and the shell then filled. with the wax. In either care bein taken, however, that the escape shall not' so free as to permit the metal to case the wax is allowed to escape through a a suitable hole at one or both ends of the dies,

'blade 10, holes are punched, such as 11,

12, etc., the metal of said holes being either forced inwards to a slight extent, as indicated at 11, or countersunk by cutting away,

as indicated at 12. Transverse studs, such as 13, 14, 15, and 16, are inserted in these holes and secured in position with their ends flush with the external surface of the blade. In the case of the studs indicated at 13, and 14, the intention is to upset the ends of the stud and weld them to the metal of the blade. In the case of the studs indicated at 15, and 16, an internal distance piece, such as 17, or 18, is provided, and the ends of the stud -may then be merely upset without welding. Injboth instances, however, the result is substantially the same, since the devices constitute both extension and compression members.

Returning now to the diagrammatic views of Figs. II, III, and IV, ,the first point to be noted is that the hollow blade is integral throughout substantially the whole of its longitudinal extent, only the transverse portion at, and immediately adjacent to,.thc

upper end in said figures, having a seam,

which may be closed-either by welding or.

by rivets. A further point is that the wall is symmetrically tapered from the base to the outer end, the character and progress of the taper being predeterminable. A further point is' that at any given cross-section, the thickness of the wall may be made uni form throughout the entire periphery. A further and highly important characteristic is that, irrespective of the feature of twist, the blade, as a whole, is longitudinally curved with relation to a plane radial tothe axis of the propeller shaft. Preferably the tip of the blade is curved forward in the direction of flight, as shownl In Fig. III, such radial plane is indicated by the line w.

The center of gravity of the blade, at each" cross section, lies in a curved line y, which I will term the median line of gravity, whichv diverges progressively from the radial plane of the line 00. Said median line of gravity may readily be determined for practical purposes by constructlng a series of short annular sections, or even solid templets, corresponding to progressive sections of the blade at predetermined longitudinal intervals, and then ascertaining the center of gravity of each scetion or templet. The median line of gravity will run (longitudinally with the blade) through these centers consecutively.

The departure of the median line of gravity from the true radial line is to be determined, in any given instance, by relation to the following factors :The contemplated axial air thrust, or pressure to be exerted against the propeller blade in the direction ,of the axis. of rotation, is known, and the contemplated total amount of centrifugal force due to the contemplated rate of rotation is also known. The arrow in Fig. III indicates the direction of flight and axial air thrust.

Other things being equal, the coefficient of air thrust will vary progressively, in a known ratio, with the rate of linear travel of the blade at any given radial distance from the axis. Such thrust tends to distort the blade mainly in a direction approximately parallel to the axis of rotation, and, ignor ing the effect of centrifugal force, to be hereinafter explained, this distortion would only be limited by the resistance of the material to deformation.

Referring now to the effect of centrifugal force, if the median line of gravity lies in a radial plane perpendicular to the axis of rotation, the total stress of centrifugal force may be considered as exerted wholly in a radial direction. To the extent, however, that the median line of gravity departs from said radial plane, the stress due to centrifugal force may be resolved, i. e., one component is exerted in the direction of the median line of gravity, the other component in a direction at right angles thereto, or substantially parallel to said axis, the tendency of this latter component being to restore the median line of gravity of the blade to atruly radial position.

Within certain limits of practical accuracy, it may, therefore, be said that whilstthe departure of/the median line of gravity from a radial plane perpendicular to the axis of rotation is attended progressively by the increase of the axial component ofcentrifugal force, such departure does not substantially affect that factor of air thrust, which is exerted in an axial direction.

Supposing, therefore, that the median line of gravity of the blade should be disposedin a radial plane perpendicular to the axis, the distortion of the blade due to axial air thrust, must proceed to a substantial extent before the axial component of centrifugal force would be practically manifested, and hence,the material of the blade would be under disruptive strain, without counterbalance by the axial component of centrifugal force, until the distortion of the blade under axial air thrust had progressed (with consequent disruptive strain) to a position 'to develop the requisite increase of suflicient departure from a radial plane of the axial component'of centrifugal force. 4

If, however, the median line of gravity of the blade be initially curved away from the radial plane in, the same direction as that in which the axial air thrust is exerted, the axial component 'of centrifugal force may be made to become effective at once, whilst the total air thrust will no greater than before.

Hence, a condltion of practical counterbalance between axial air thrust on the one hand, and the axial component of centrifugal force,-on the other hand, may be established as soon as the predetermined rate of rotation has been attained, and thus the normal operative position of the blade, at such 'rate of -rotation, may be such as to minimize the disruptive strain which would otherwise be. due to axial air thrust.

Under the conditions present in a blade consisting of a hollow metallic shell, it is possible and this peculiarly important that the disruptive strains should be counterbalanced as far'as may be attained by disposing the median line of gravity in a curve which progressively departs from the radial plane in the same direction as that in whichthe axial air thrust is exerted, the extent and progressiverate of departure being determined by reference to'the intended factors above set forth for the conditions of actual service, and by the dimensions and other structural characteristics of theblade.

Hence, no more specific building directions can be given than are comprised in the foregoing statement from which any one of the faccan readily and can give to said line that curvature which shall best establish the desired counterbalance of axial air thrust by the axial component of centrifugal force.

Having determined this curvature, the die cavities should be so constructed as to dis-- pose the metal of the shell, as far. as is practical, symmetrically with relation to the median line of gravity.

Referring-now to Figs. V, and VI,'I embody these in thepresent application in order to illustrate a convenient organization of means for mounting my propeller blade upon the driving shaft. In said fig-- ures, 20 represents the hub of-thepropeller shaft, which carries 1n this instancetwo diametrically located extend across the interspace between, the

Upon these seats, the bottom .flanges 2, of the propeller blades, are secured by means of a series of bolts 25', and nuts 26, which annular seats 21, and 22.

two seats 21, and 22, so that a pair (if diametrically arranged blades may be detachably mounted and adjustably secured with a maximum of stability. I c Having thus described my invention, I claim:

1. A hollow metallic propeller blade of gradually diminishing thickness from the base to its tip, said blade being longitudinally curved with relation to a plane radial to its axis of rotation and having the'metal of its wall at successive cross sections symmetrically disposed to afford a continuously 7 symmetrical median line of gravitywhich departs angularly from a plane radial to the axis of rotation, the direction of de-= parture being similar to that in which the I axial air thrust is exerted, and the extent of departure being such that at a predetermined normal rate of rotation, the axial component of centrifugal force shall sub-- 'stantially counterbalance the axial air thrust.

2. .A hollow metallic propeller blade of gradually diminishing thickness from the base to its tip, said blade being longitudi nally curved with relation to a plane radial to its axis ofrotation and having the metal of its wall at successive cross sections symtrically disposed to afford a continuously symmetrical median line of gravity which/ departs, in a progressively increasing curve, from a plane radial to the axis of rotation, 9

the direction of departure being similar to to its axis of rotation and having the metal of its wall at'succes'sivecross sections symmetrically disposed to afford a continuously symmetrical median line of gravity, in combination with stay pieces extending between the sides thereof, the respective stay piecesbeing of such weight and so' located with relation to the median line of' gravity of the blade itself, as to avoid substantial displacement or distortion of said line.

In testimony whereof, I, have hereunto signed my name at Philadelphia, Pennsylvania, this ninth day of March, 1918.

v J OHN" W. SMLLTH.

Witnesses:

JAMES H. BELL,

-. E. L. Fumnn'ron, 

